I have started looking at this, starting with the iTunes classical fields. One thing to be aware of is for reasons only known to the iTunes developers, for MP3 files, they put the Work name into the ID3 grouping tag (TIT1) which MB is already using for the "Grouping" field. For M4A files, they have created a separate tag.
Also for MP3 files, for some reason they dont save a tag for "Show Movement".
The approach I am taking is to maintain consistency with Mp3Tag, ie. new dedicated tags: TXXX/WORK and TXXX/SHOWMOVEMENT for Mp3 files, so that means anyone using iTunes, the tagging wont carry across.
For M4A the tagging will be completely consistent with mp3Tag and iTunes.
I should have this aspect of classical music support done today